The Eagle Lake volcanic field occupies the junction of the Cascades, Sierra Nevada, and Basin and Range geologic provinces and consists of 15 cinder cones and basalticlava flowvents within a larger Quaternary basaltic field. The vents are aligned along faults defining the Eagle Lake volcano-tectonic depression, and are the southernmost example of late Quaternary back-arc spreading in the northwestern Great Basin. The latest long-lived eruptive period has been roughly estimated to have occurred about 50,000-100,000 years before present (Grose, in Wood and Kienle 1990). Miller (1989) mapped four Holocene centers in the Eagle Lake volcanic field.